The invention is concerned with security paper for bank notes and the like, and to processes for producing such paper.
It is widely known to use in bank notes security strips or threads which are made from a transparent film provided with continuous reflective metal layer, vacuum deposited aluminium on polyester being the commonest example. Bank notes made from such paper have been in general circulation in many countries for many years. The principal feature of such threads is that when they are embedded in security paper, and the paper is subsequently printed to provide the security document, which includes bank notes, the thread cannot be readily discerned in reflected light but is immediately apparent as the security thread provides a dark image when the document is viewed in transmitted light. Such well-used security threads are simple in concept, but the device is extremely effective since the optically variable effect which is provided cannot be accurately simulated, for example by printing a line on the paper. The continued wide-spread use of this security feature for many years after its first introduction is a strong indication of its efficacy.
It is known that metallised films can be produced such that no metal is present in controlled and clearly defined areas. Such partly metallised film can be made in a number of ways. One way is to selectively demetallise regions using a resist and etch technique such as is described in U.S. Pat. No. 4652015. Other techniques are known for achieving similar effects; for example it is possible to vacuum deposit aluminium through a mask or aluminium can be selectively removed from a composite strip of a plastic support and aluminium using an excimer laser.
It is important to provide even better security paper in order to enhance the public perception/recognition of a security thread for the purpose of authentication by the public. This may be achieved by incorporating within the security thread a pattern which is readily recognisable by the public while retaining the widely accepted optically variable effect which is provided by the continuously metallised strip. Many currencies of the world incorporate a microprinted thread incorporating fine printed characters but these are generally too small to be recognised in a printed document with the unaided eye. If the printed characters are made larger, they are very obvious when viewed in reflected light and readily simulated by a counterfeiter. The present invention makes use of a pattern or characters sufficiently large so as to be readily perceived by the naked eye and formed from a partially metallised strip such that the strong optically variable effect of a continuously metallised strip is essentially retained.